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Using Averages To Bend the Uncertainty Principle

Soulskill posted more than 3 years ago | from the i-hear-heisenberg-was-a-good-cook dept.

Science 112

summerbreeze writes "Researchers at the University of Toronto have conducted a two-slit experiment, published in Science, that uses 'weak measurement' on photons to push back the boundaries of what can be known about them, given the Heisenberg Uncertainty Principle. Jason Palmer does a great job reporting this experiment to us mere mortals in a BBC article: 'The team allowed the photons to pass through a thin sliver of the mineral calcite which gave each photon a tiny nudge in its path, with the amount of deviation dependent on which slit it passed through. By averaging over a great many photons passing through the apparatus, and only measuring the light patterns on a camera, the team was able to infer what paths the photons had taken. While they were able to easily observe the interference pattern indicative of the wave nature of light, they were able also to see from which slits the photons had come, a sure sign of their particle nature."

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first post (-1)

Anonymous Coward | more than 3 years ago | (#36337672)

first post, posting as AC so I dont get flamed...

Another one!?!?! (3, Interesting)

farrellj (563) | more than 3 years ago | (#36337674)

Yeah Canada, again!

Canada certainly does punch above it's weight in many areas...

But this is a really interesting experiment! It really does turn the classic double slit experiment on it's ear!

Re:Another one!?!?! (-1)

Anonymous Coward | more than 3 years ago | (#36337956)

"Canada certainly does punch above it's weight in many areas..."

We also know how to use apostrophes and the difference between contractions and possessives.

Re:Another one!?!?! (1)

Have Brain Will Rent (1031664) | more than 3 years ago | (#36338118)

But we're usually too polite to mention it when someone makes a mistake.

Re:Another one!?!?! (2)

beckett (27524) | more than 3 years ago | (#36340574)

sorry.

Re:Another one!?!?! (1)

Have Brain Will Rent (1031664) | more than 3 years ago | (#36338046)

By again do you mean the Sudbury Ontario students winning the NASA Moon Robot competition?

I don't get it (1)

Anonymous Coward | more than 3 years ago | (#36337676)

How does this differ from the classic two-slit experiment?

Re:I don't get it (3, Informative)

ductonius (705942) | more than 3 years ago | (#36337708)

In the classic experiment, if you try to find out which slit the photons are going through, they stop behaving as waves.

In this experiment, they can know which slit the photons when through, but still get the light to behave as a wave.

Re:I don't get it (2)

Joce640k (829181) | more than 3 years ago | (#36337744)

I thought the photons went through both slits.

(and also took a detour around the Horsehead Nebula along the way...)

Re:I don't get it (5, Insightful)

OeLeWaPpErKe (412765) | more than 3 years ago | (#36337914)

That's the idea of quantum physics : particles or waves don't move on any specific path, they move on all possible paths between 2 points. But once anything interacts with them the "potential history" function collapses, and they have taken one specific path, which had only one specific set of events taken place.

So photons only go through both slits in the function that describes their movement, not in reality. It's just that the only way to describe their behavior is to assume they go through both slits, because we can't measure these things without disturbing them.

Why not ? Well imagine you have to determine if it's the national holiday in India (they have a big elephant parade). But you don't actually have any tools smaller than elephants to measure this. So every hour or so you catapult an elephant into the main street of New Delhi, and you see if the elephant hits the detector you've set up at the other end of that street. Obviously any "detected" elephant will not be unaffected, and won't ever get to the place where the parade elephants normally end up, and your interference pattern will be gone. Now s/elephants/photons/ and you have the problem of quantum physics (and yes this is a simplification).

Now what these scientists did is they place an "elephant guide" (say a slide) in front of one of the two slits, which does not really affect the elephants, but it does alter their path a little bit, and this is reflected in the position the elephant hits the plate behind the detector. Now they know (not for certain, but better than 50%) which slit the elephant went through, yet they have managed to avoid totally destroying the normal path the elephants take, so the elephants from both slits are still in a position to interact.

A (very) nice video about this : http://www.youtube.com/watch?v=DfPeprQ7oGc [youtube.com]

Best Analogy Ever (2)

Oxford_Comma_Lover (1679530) | more than 3 years ago | (#36337938)

> Why not ? Well imagine you have to determine if it's the national holiday in India (they have a big elephant parade). But you don't actually have any tools smaller than elephants to measure this. So every hour or so you catapult an elephant into the main street of New Delhi, and you see if the elephant hits the detector you've set up at the other end of that street. Obviously any "detected" elephant will not be unaffected, and won't ever get to the place where the parade elephants normally end up, and your interference pattern will be gone. Now s/elephants/photons/ and you have the problem of quantum physics (and yes this is a simplification).

You just described quantum physics... with elephants.

Excellent.

Re:I don't get it (0)

boristhespider (1678416) | more than 3 years ago | (#36338144)

Fuck me. Someone with mod points boost this, it's about the clearest explanation of quantum physics I've seen - including four years of lectures. The amount of people who babble about "observation" apparently seriously believing that "observation" is important is startling. Your first two sentences summarise everything clearly and neatly and without any extra bullshit.

Then you carried on to use elephants, which is only even more laudable.

I once wrote a similar proposal for running a multi-slit experiment using gerbils; since the de Broglie wavelength of a gerbil is so short it would have to have *exceptionally fine* slits. I also added that if animal protection agencies kicked up a stink we could use human slaves instead, since if we can make a diffraction grating fine enough for gerbils we can refine the process only a small amount further and make one for humans. We would, of course, need a trench dug behind the slit to collect the diffraction pattern.

The lecturer seemed quite amused, which surprised me because I did it to piss him off. He stuck it outside his office for the rest of that semester.

Re:I don't get it (-1)

Anonymous Coward | more than 3 years ago | (#36338242)

you really sound like an annoying, intolerable faggot

Re:I don't get it (1)

boristhespider (1678416) | more than 3 years ago | (#36338250)

I thought that was obvious from the start.

Re:I don't get it (0)

Anonymous Coward | more than 3 years ago | (#36338762)

No, he doesn't; you do.

Have a Nice Day.

Re:I don't get it (0)

Anonymous Coward | more than 3 years ago | (#36342246)

people who use semicolons in sentences are annoying, intolerable faggots

Re:I don't get it (5, Informative)

Old Wolf (56093) | more than 3 years ago | (#36338926)

Sorry to burst some bubbles, but I believe this analogy is not correct :( In fact it is not really possible to analogize quantum mechanics with anything classical, which is what people are getting at when they say that nobody really understands it.

In the experiment in TFA, they never found out which slit any particular photon went through. They have only collected some data about the average behaviour of the total set of photons. TFA suggests the scientists gathered a statistic somewhat like "X photons went through slit 1 and Y photons went through slit 2". Even here, I do not believe this is correct as I have worded it. I haven't read the paper at the article is based on, however if we follow the explanation of the first paragraph of your post, we will have an interference pattern that looks a bit different to the 50-50 one, where the possible paths between the two points have a greater 'density' of going through one of the particular slits. I would imagine that as you gradually change this ratio from 50-50 through to 0-100 the pattern would morph until it ended up being a one-slit diffusion pattern.

The rest of your post makes the same mistake as early efforts to explain the 'uncertainly principle', which was initially thought to be something like: "The particles have exact positions and momenta, but any attempt to measure them must disturb the system'. It was fairly quickly found that this was wrong, and the particles actually do not have well-defined positions and momenta (this is implicit in Schrodinger's equation and other such equations, the 'uncertainty principle' just describes a fact of the mathematical description of what a wavefunction is).

So photons only go through both slits in the function that describes their movement, not in reality.

  Certainly, photons behave according to the function that describes their movement. However, what is 'reality' is an open question (this is known as the interpretation of quantum mechanics). Some interpretations say that the photon travels through one slit but we cannot know which; some say that the function describing their movement *is* reality, and some say that 'reality' only consists of the photon's emission and its detection; not the stuff in between.

Re:I don't get it (4, Insightful)

Savantissimo (893682) | more than 3 years ago | (#36339280)

That's much better than the original explanation. To boil it down even further, quanta are waves when they are going somewhere (propagating) and particles when they get there (interacting). Each photon does actually go through both slits, which isn't a problem because it's a wave. When it hits the screen, it interacts in an all-or nothing, localized fashion, which gives the appearance of a particle.

The interesting thing about this experiment is that it further demonstrates that there is a continuum between particle and wave, interaction and propagation, but that this can only be shown as a statistical effect using many observations.

Re:I don't get it (0)

Anonymous Coward | more than 3 years ago | (#36339350)

Was it that the detector collapsed some of the photons completely, or all of the partially?

Re:I don't get it (0)

Anonymous Coward | more than 3 years ago | (#36338990)

So photons only go through both slits in the function that describes their movement, not in reality. It's just that the only way to describe their behavior is to assume they go through both slits, because we can't measure these things without disturbing them.

So why is there a interefence pattern even when they are shot one at the time? they must interfere with themselve.

Re:I don't get it (1)

Savantissimo (893682) | more than 3 years ago | (#36339294)

You are correct, the OP wasn't. The wave goes through both slits, but the measurement of an individual photon must be discrete and thus particle-like.

Re:I don't get it (1)

Guignol (159087) | more than 3 years ago | (#36341068)

There is no interference pattern when you shoot just one. it does not need to interfere with itself.
The 'wave' part of the particle is not so much 'the particle itself taking the form of a wave', it's the probability amplitude that if you measure it, it will be in this or this particular place.
the interference pattern only appears when you shoot many many many particles, so, in a way, you could think about the particles interfering with themselves in the past and the future which is even weirder, but that's not what happens. The interference comes from the probability wave, and it's nothing short of amazing

Re:I don't get it (1)

Flodis (998453) | more than 3 years ago | (#36342608)

You're wrong. There is a pattern even in the single-shot experiment.

Re:I don't get it (1)

Flodis (998453) | more than 3 years ago | (#36342614)

A bit too trigger happy. Sorry about that.

Re:I don't get it (1)

Anonymous Coward | more than 3 years ago | (#36339830)

Why not ? Well imagine you have to determine if it's the national holiday in India (they have a big elephant parade). But you don't actually have any tools smaller than elephants to measure this. So every hour or so you catapult an elephant into the main street of New Delhi, and you see if the elephant hits the detector you've set up at the other end of that street. Obviously any "detected" elephant will not be unaffected, and won't ever get to the place where the parade elephants normally end up, and your interference pattern will be gone. Now s/elephants/photons/ and you have the problem of quantum physics (and yes this is a simplification).

Sure...everything is simpler once you've explained it in terms of catapulting elephants into New Delhi. Wait..what?

Re:I don't get it (1)

matunos (1587263) | more than 3 years ago | (#36339872)

So photons only go through both slits in the function that describes their movement, not in reality. It's just that the only way to describe their behavior is to assume they go through both slits, because we can't measure these things without disturbing them.

This doesn't explain the single-electron version of the double slit experiment, in which an interference pattern emerges, demonstrating that the electron wave function interfered with itself and thus must have passed through, wave-like, both slits at the same time. It's only when you try to observe which slit the electron goes through that it dutifully fulfills our expectations and goes through both. The experiment discussed above apprarently reveals a way to do some level of observation without completely collapsing the wave functions, or all of the wave functions, or of restoring the wave function post-observation, I guess.

bad analogy (2)

thunderclap (972782) | more than 3 years ago | (#36339946)

After getting up from laughing so hard, I will say this: "Well imagine you have to determine if it's the national holiday in India (they have a big elephant parade). But you don't actually have any tools smaller than elephants to measure this. So every hour or so you catapult an elephant into the main street of New Delhi, and you see if the elephant hits the detector you've set up at the other end of that street." What in reality you will actually get is: a: Dead elephants. B: crushed people, cars, wholes in buildings, blood and carcass everywhere. You will never get whether its an national holiday. You will get very angry animal rights people and eternal hatred. And you will hit the detector only once.

Re:I don't get it (2)

SpiralSpirit (874918) | more than 3 years ago | (#36340864)

your understanding of the HUP is not correct. It isn't that measuring the elephants disturbs them, it's that quantum elephants have trajectory and position, and the more you know of one the less you can possibly know about the other. When you measure one variable, the more exactly you measure it, the less you know about the second variable. It's not that the instrument disturbing the particle that creates uncertainty. For a better explanation, involving dogs and rabbits, I recommend "How to Teach Quantum Physics to Your Dog" by Chad Orzel. Even relatively simply put, there will be parts of the experiments discussed that most people will have to read more than once to at all get even a basic, dog-like understanding.

Re:I don't get it (5, Funny)

migla (1099771) | more than 3 years ago | (#36337752)

The key here is surreptitiousness. The researcher must act uninterested and as if they aren't trying to measure anything in particular and especially not with any fine accuracy. It helps if they whistle and distractedly reorganize bottles on a shelf while glancing fleetingly over at the experiment letting out a bored "Meh" as they do so.

Re:I don't get it (1)

Anonymous Coward | more than 3 years ago | (#36337878)

It's why we haven't found the Higgs boson, we care about finding it too much.

Re:Higgs (2)

TaoPhoenix (980487) | more than 3 years ago | (#36339044)

The same attitude is needed to look at Higg's Wife's bosom without getting punched.

Re:I don't get it (1)

Sulphur (1548251) | more than 3 years ago | (#36337902)

The key here is surreptitiousness. The researcher must act uninterested and as if they aren't trying to measure anything in particular and especially not with any fine accuracy. It helps if they whistle and distractedly reorganize bottles on a shelf while glancing fleetingly over at the experiment letting out a bored "Meh" as they do so.

Yes, but they are planning to do so.

Re:I don't get it (2, Informative)

Anonymous Coward | more than 3 years ago | (#36337986)

Quantum mechanics is a statistical theory, valid only in the statistical limit of an infinite number of measurements and looking at the ensemble. It actually places no inherent limits on a single measurement, only on an ensemble of measurements. Hence, you have no violation of the uncertainty principle because you are tracking individual photons or a very small number of them. The Stern-Gerlach experiment back in the day observed individual particle strikes but when viewed as a large average you had the interference pattern characteristic of wave phenomena, while the individual flashes on the phosphor screen indicated a particle nature.

Only people who do not understand quantum mechanics (and they are legion) forget that it is a statistical theory and go off on tangents about Schrodinger's Cat (a severe criticism of an interpretation of QM that was once fashionable) and the like any more.

The interest of this experiment is that they succeeded in finding an apparatus which was capable of going below the Heisenberg limit--recall that Heisenberg posed many measurement examples where there would be perturbations of the measurement process at least as large as the limits imposed by the (statistical) uncertainty principle. In this sense, it is a sort of confirmation that QM is a statistical theory and describes the outcomes of measurements, and need not describe nature directly (in the same sense that you do not need to understand the mechanics of dice in order to predict the probabilities for the next throw).

Re:I don't get it (3, Interesting)

Intron (870560) | more than 3 years ago | (#36338140)

Quantum mechanics is a statistical theory, valid only in the statistical limit of an infinite number of measurements and looking at the ensemble. It actually places no inherent limits on a single measurement, only on an ensemble of measurements. Hence, you have no violation of the uncertainty principle because you are tracking individual photons or a very small number of them. The Stern-Gerlach experiment back in the day observed individual particle strikes but when viewed as a large average you had the interference pattern characteristic of wave phenomena, while the individual flashes on the phosphor screen indicated a particle nature.

That's absurd. The interference patterns in the two-slit experiment are still created even when the intensity is reduced to the point that there is never more than one photon traversing the slits at a time. The QM rules apply to every wavicle, not just to aggregations.

You are misinterpreting Stern-Gerlach which also shows that each particle has quantized values for angular momentum and hence meets QM predictions.

Re:I don't get it (2)

Old Wolf (56093) | more than 3 years ago | (#36338952)

That's absurd. The interference patterns in the two-slit experiment are still created even when the intensity is reduced to the point that there is never more than one photon traversing the slits at a time. The QM rules apply to every wavicle, not just to aggregations.

I think you misunderstand the post you're replying to. For each photon fired in the two-slit experiment, the photon can register at ANY point on the detector -- that's a fact. It is only once we have fired many photons that we find fewer photons registered in certain areas and more photons registered in other areas, which cannot be explained in classical terms.

Re:I don't get it (1)

Shippu (1888522) | more than 3 years ago | (#36340320)

Can the first one register anywhere?

Re:I don't get it (2)

Savantissimo (893682) | more than 3 years ago | (#36339390)

I think you have that backwards. QM places strict limits on the information obtainable from individual measurements, but much less strict limits on measurements of ensembles. Any individual interaction can only yield so much information, many interactions can yield more information - but each interaction is separate and doesn't technically say anything about any of the other individual interactions, but rather about the process producing the interactions. The Heisenberg uncertainty principle (specifically the time-energy form) applies even to classical waves - I've seen it myself in testing tunable-Q filters - no need for particles at all. Also, ensembles are not needed for a rigorous statistical theory - the Bayesians deride those who think that ensembles are the basis of statistics as "frequentists", and instead say that probability is really a measure of observers' information, and has different values depending on the degree of information each observer has. The "statistical distribution" of the frequentists is not an objective thing, but depends on the degree of information the observer has, and changes as more data comes in. The Bayesian approach meshes much better with information theory and QM and easily explains apparent paradoxes such as the Monty Hall problem.

Re:I don't get it (0, Informative)

Anonymous Coward | more than 3 years ago | (#36338448)

Glad your comment is modded funny. But it is *hugely* irritating how many people seem to think quantum "observation" has anything at all to do with humans.

The philosophers like to state that quantum physics say that if no-one's around to hear a tree fall in a forest, the tree doesn't fall. That's bullshit, of course. To avoid observation, the falling tree would have to avoid hitting anything at all (even a single photon hitting it would end it's superposition).

Now the question a lot of physicists really have : how do you get idiot apes to actually believe that they are not any more special than any microbe or boring particle of dust lost between galaxies ?

Re:I don't get it (2)

Old Wolf (56093) | more than 3 years ago | (#36338980)

The single photon wouldn't necessarily end the superposition, it would just entangle the tree with the environment (which it already was anyway).

Obviously it's absurd to suggest that humans cause collapse, but nobody has yet suggested a convincing enough explanation of what does cause collapse (or if collapse even happens at all), so I don't think the tree is dead just yet :)

Re:I don't get it (2)

Savantissimo (893682) | more than 3 years ago | (#36339444)

"...the falling tree would have to avoid hitting anything at all (even a single photon hitting it would end it's superposition)"
No, , (assuming the timing of the fall of the tree has some uncertainty, which it must) the tree would just entangle whatever it interacted with into that superposition between fallen and not-fallen and as those things in turn interacted with other things there would be an outward spreading wave of entanglement that would quickly become effectively irreversible (decoherent). In one set of paths the environment (including humans) would be indirectly entangled with a standing tree, in the other with a fallen tree. Decoherence is a matter of degree - for slight degrees it can be reversed, the "observation" undone - see the "quantum eraser" experiments.

Re:I don't get it (1)

NotAGoodNickname (1925512) | more than 3 years ago | (#36337758)

I thought the point of the classic experiment was that you couldn't tell which slit they went through, but they were obviously particles because of the impressions they made on the photo-sensitive paper, but they also obviously traveled in waves because of the interference pattern they made at the end of the experiment.

Re:I don't get it (-1)

Anonymous Coward | more than 3 years ago | (#36337832)

Shows what you know, doesn't it?

Re:I don't get it (2)

Scrameustache (459504) | more than 3 years ago | (#36337874)

obviously particles because of the impressions they made on the photo-sensitive paper

No, that's got nothing to do with being a particle or not. The fun of this experiment is that it shows light to be a wave (because of the interference pattern) unless you measure photons going through the slits, in which case there is no interference pattern. Also works with electrons, btw.

I also don't get it (0)

Anonymous Coward | more than 3 years ago | (#36337760)

But where does the interference come from, if the individual photon does not extend through both holes? They are using single photons.

Re:I also don't get it (0)

Anonymous Coward | more than 3 years ago | (#36337940)

lol

Re:I don't get it (2)

Scrameustache (459504) | more than 3 years ago | (#36337850)

In this experiment, they can know which slit the photons when through

They infer through a study of averages.

Re:I don't get it (1)

MaskedSlacker (911878) | more than 3 years ago | (#36337756)

You can't measure both variables in the traditional double slit experiment--either you measure which slit the photons go through, or you detect an interference pattern, not both. It was one of the critical pieces of evidence in favor of the Copenhagen interpretation of quantum mechanics. Measuring which slit the photons traveled through collapses their wavefunction to a position eigenstate, changing their wavefunctions so that no interference pattern is created.

From the summary it sounds like they measured position (which slit the photons went through) in a way that did not completely collapse the wave function, but only collapsed it enough to distinguish which slit the photon's must have gone through, thus not eliminating the interference pattern. That's my best guess, but it's been years since I've taken quantum mechanics (and who knows how inaccurate the summary is).

Re:I don't get it (1)

Joce640k (829181) | more than 3 years ago | (#36337864)

I thought the entire point of the experiment was that you still get interference with single photons, ie. they go through both slits.

Saying they measured which slit they went through doesn't make any sense if they go through both.

Re:I don't get it (2)

MaskedSlacker (911878) | more than 3 years ago | (#36337958)

Eh, hard to explain what I mean without drawing graphs of wavefunctions, but I'll try (and I may be wrong anyway, someone who's done QM past two 400 level classes four years ago would have to weigh in there).

The interference pattern isn't the result of the photons going through both slits per se (that's a really awkward, but accessible way of explaining the math, and I don't think it works very well), but a result of the wavefunctions of the photons from each slit overlapping and interfering with each other. When you measure which slit the photons travel through the position wavefuction (x-hat Psi) collapses to a Dirac Delta function (basically an infinite probability spike at x=x_0, where x_0 is the position of the slit). Because of this the wavefunctions from the photons going through different slits no longer overlap, so they no longer interfere with each other. The summary's use of the term 'weak measurement' suggests to me that they measure particle position in a way which did not collapse the wavefunctions to a dirac delta entirely, but only enough so that the probability that a photon went through one slit and not the other is large (I'd have to read the actual article to be more specific than that). It is then conceivable that the wavefunctions would still overlap and interfere with each other.

Essentially, they aren't measuring which slit the photons go through exactly, they're taking a measurement which of slit the particles very, very likely went through. For each photon they're saying something like "there's a 99% chance it went through the left [or right] slit," and that measurement apparently doesn't destroy the interference pattern.

Or I could be entirely misreading the summary, or the summary could be fubar.

Re:I don't get it (2)

Soft Cosmic Rusk (1211950) | more than 3 years ago | (#36337980)

The point is that if you measure whether or not the photon went through slit 1, you "force it to take a stand", and choose which slit to go through. Thus, the wave function collapses and you no longer get the interference pattern, but just two blops of photons on the back wall.

Re:I don't get it (4, Interesting)

Joce640k (829181) | more than 3 years ago | (#36338000)

The fun thing is that you can do this with photons which were gravitational lensed around both sides of a galaxy and *still* collapse the wave function. Your measurement instantly changes something which happened a billion years ago (the lensing).

Re:I don't get it (1)

Soft Cosmic Rusk (1211950) | more than 3 years ago | (#36338156)

Yeah, okay, so I wasn't completely stringent in my choise of words, but I think the point got through ;)

Re:I don't get it (1)

Anonymous Coward | more than 3 years ago | (#36339522)

The fun thing is that you can do this with photons which were gravitational lensed around both sides of a galaxy and *still* collapse the wave function. Your measurement instantly changes something which happened a billion years ago (the lensing).

Because World, wait for it ... wait for it ... we live in a simulation. The only way you can affect the past is by altering the initial values at run-time. COmon people, quantum mecanics and information theory explains god and bible and where are we gonna go. heaven is the simulation of all our brains subconcious mind. thats why we are connecting at bigger scale more and more, the invention of internet, mother of connectionhood is the natural pathway. someday all our brains will be somehow be linked togheter and toghtere we`ll create the implanted seed of heaven in our minds. hence the manipulative nature of humans.

Or did I just smoked some weed?
Weird, I did.

Try this high,
http://critical-path.itgo.com/Articlesanscover.html

Re:I don't get it (0)

Anonymous Coward | more than 3 years ago | (#36341788)

I can finally rejoice with my guinea pig again as his brain would be wired into heaven as well and we could have a one to many orgasm with everyone at the same time, you crackhead!

Re:I don't get it (2)

ShakaUVM (157947) | more than 3 years ago | (#36339552)

>>The fun thing is that you can do this with photons which were gravitational lensed around both sides of a galaxy and *still* collapse the wave function. Your measurement instantly changes something which happened a billion years ago (the lensing).

Even weirder? You can undo the wavefunction collapse, so the photon that you were about to measure appears instead two billion light years away.

http://en.wikipedia.org/wiki/Delayed_choice_quantum_eraser [wikipedia.org]

Re:I don't get it (2, Funny)

Anonymous Coward | more than 3 years ago | (#36339730)

I don't get it How does this differ from the classic two-slit experiment?

Well, the explanation might be a bit long, but try to bare with us.

You see, the main difference is

#include <article.h>

See? Simple as can be!
Sorry if that post got too long winded...

Re:I don't get it (1)

MillionthMonkey (240664) | more than 3 years ago | (#36341458)

How does this differ from the classic two-slit experiment?

Well you can't tell from the article, so I'm guessing based on the abstract.

From what I gather about this, think of what calcite does. It exhibits birefringence [tinyurl.com] , meaning it has different indices of refraction for photons polarized vertically vs. horizontally. You can see that with the way unpolarized light in the room gets deflected by a piece of calcite if you put it on the page of a book, when you see two images of every letter.

So I think what they did here was the classic two slit experiment, with three differences. First, each slit gets a polarizer placed in front, with the left and right slits each getting a polarizer that is oriented perpendicularly to the other one. Now, for a given photon to get through one slit or the other, it can only have one polarization vs. the other. But you haven't yet destroyed the interference pattern since you didn't measure its polarization, so the uncertainty of which slit remains, and the ripples showing on the screen should still show.

Second, the sheet of calcite is placed behind the slits, so if a photon went through the right vs. left slit, we can see that when the calcite displaces it clearly in one of two directions based on that. Now we know what the polarization was when it hits the screen, when it hits one of two separate lines now showing there. And there are no ripples in each of these two lines now [guessing confidently], since we are making a really strong measurement of position at time of slit passage. As if we were blocking one of the slits.

Third, you only take a picture of the resulting pattern with a camera, having a long exposure time.You record where it hit, but you don't know which one hit there. You only have a probability distribution of where any given photon might have struck the screen. [Think, "I am Spartacus!"]

This is making a "weak" measurement for each given photon, because you're only talking about the average of all photons going through these slits. And so what you see on the screen is what you were seeing before, the two lines separated, but now with no knowledge of photon identity. You shined a lot of photons through the apparatus without possibly having any knowledge of which one went where, like you might somehow while looking at a screen all night with fast eyes or instruments.

What they are reporting is that the ripples return when you use this measurement-weakening time-lapse photography procedure [guessing confidently again]. That's why these Canadians will see the ripples go away on a scatter plot if they slow this thing down to a stream of single photons so that they can carefully watch where each photon hits on the screen. The use of a camera and only walking into the room after taking the picture is critical if you intend to see interference patterns.

What I never understood about the uncertainty p. (2)

StripedCow (776465) | more than 3 years ago | (#36337710)

So, as I understand it, the uncertainty principle tells us that in order to determine the position of a particle, we'd have to make a photograph of it using a sufficiently high frequency of light, otherwise we'd get a severe interference pattern. However, this high frequency of photons is coupled to high energy, thus knocking the original particle out of its path (in other words changing its momentum). So far so good.

However, assume that the particle is perfectly symmetric, e.g. a sphere. Then the interference pattern will also be symmetric. The image we'd get by making a photograph would look like a bunch of concentric circles. Where is the original particle? Well, at the center of those cirlces of course!

So this is what I don't understand. We can actually deduce the position of the particle precisely from the interference pattern. So where is all the fuzz coming from?

Re:What I never understood about the uncertainty p (1)

John Hasler (414242) | more than 3 years ago | (#36337732)

> So, as I understand it...

You don't. Please read up on it.

Re:What I never understood about the uncertainty p (1)

Anonymous Coward | more than 3 years ago | (#36337942)

"I think I can safely say that nobody understands quantum mechanics."

- Richard Feynman

Re:What I never understood about the uncertainty p (1)

Soft Cosmic Rusk (1211950) | more than 3 years ago | (#36338180)

But it IS possible to understand the math behind the uncertainty principle, even though the OP obviously doesn't.

Re:What I never understood about the uncertainty p (0)

Anonymous Coward | more than 3 years ago | (#36341800)

Yeah neither do you, which is why your reply is two words long. Either that or you're just an a-hole. Be helpful or be quiet.

  The original questioner may have to "read up on it" but methinks he'll see you at the study table right next to him.

Re:What I never understood about the uncertainty p (0)

waives (1257650) | more than 3 years ago | (#36342494)

just because we understand it does not obligate us to educate every moron spewing complete drivel.

take some initiative and educate yourself. and shut up until you do.

Re:What I never understood about the uncertainty p (4, Informative)

mmell (832646) | more than 3 years ago | (#36337768)

Um, just to ask - what particle? Oh, you mean the light wave?

You're understanding of the basic assertion of the Uncertainty Principal is correct - in order to know the exact position of a particle at an exact moment, you have to measure the particle which changes it's position. Right on.

However, when speaking of electromagnetic phenomena, it's generally understood that we're speaking of something which can be either a particle or a wave, depending upon the property being observed. Call it a 'wavicle', if you like. It's the act of measuring the behavior that "collapses the wave function" - i.e., I can demonstrate exactly where a photon struck a sensor under a certain set of conditions, but doing so collapses the wave function. OR I can demonstrate the wavelike properties of light, but only by sacrificing any clue to the position of the photons which create that wave structure (oddly enough, collapsing the wave function once again).

Now, this is only my understanding of the condition, and I'm not really that certain I've got it right . . .

Re:What I never understood about the uncertainty p (2)

similar_name (1164087) | more than 3 years ago | (#36337834)

Longtime since I thought about this, but isn't everything a wave-particle? I remember reading somewhere that even the earth has properties of wave such that it has specific orbits around the sun. As I recall the orbits only differ by millimeters (something to do with the way the wave function lines up in orbits around the sun similar to why electrons exist in certain orbits but not in between) but nonetheless the earth has wave properties too.

Re:What I never understood about the uncertainty p (1)

Unequivocal (155957) | more than 3 years ago | (#36340634)

I'm not a physicist but I'm pretty sure this is wrong. It is true that macroscopic objects are predicted to have wave functions, and some macroscopic objects have had quantum properties measured (in pretty esoteric experimental setups), but planet sized objects don't follow orbits around the sun based on their wave functions at all. I'm not even sure if you're suggesting that, but I wanted to clarify in case someone thought you were.

Wave (1)

vuo (156163) | more than 3 years ago | (#36341716)

Strictly speaking this would apply only if the Earth was a single particle. You can calculate a de Broglie wavelength for everything, but everything is not a single wave, but a composite object.

Re:What I never understood about the uncertainty p (1)

Unequivocal (155957) | more than 3 years ago | (#36340624)

Not to be pedantic (and I'm not a physicist) but you don't "sacrifice any clue to the position" you only sacrifice a precise clue. You still have a pretty darn good clue where the particle is via the quantum wave functions. It's much more likely to be near where it was emitted than far away from that spot for example - you have a statistical clue as to it's position in other words.

I'm not saying you don't know this, just wanted to clarify the language for other readers.

Re:What I never understood about the uncertainty p (4, Informative)

Inquisitus (937664) | more than 3 years ago | (#36337812)

The HUP is more fundamental than that. It doesn't just say that we can't know where a particle is because measurement disturbs it; rather it's telling you that the particle actually doesn't have a definite trajectory. In fact, it's so fundamental that it has its own mathematical formalism (commutativity of operators), upon which most of quantum mechanics is constructed.

It's important to realize that in quantum mechanics, the position of a particle is indefinite, and is specified by a diffuse/spread-out "cloud" probability, and only in special cases does this cloud collapse to a single point (which corresponds to the particle being in a definite place).

Note that it is possible (theoretically) to know the position or momentum of a particle, just not at the same time, since measuring one causes the other to become indeterminate.

Re:What I never understood about the uncertainty p (3, Insightful)

colinrichardday (768814) | more than 3 years ago | (#36339540)

that it has its own mathematical formalism (commutativity of operators) It's the commutators that matter. Let u and v be the operators for position and momentum in the same direction (plus or minus). Then the commutator is uv-vu. As the operators do not commute, the difference is not zero. Hence, we get the uncertainty principle.

Re:What I never understood about the uncertainty p (0)

Anonymous Coward | more than 3 years ago | (#36337910)

So this is what I don't understand.

Sorry, but this is not the only point you don't understand. None of the points you make in your post show any basic understanding of the issue you are trying to discuss.

Re:What I never understood about the uncertainty p (1)

Scrameustache (459504) | more than 3 years ago | (#36337928)

as I understand it, the uncertainty principle tells us that in order to determine the position of a particle, we'd have to make a photograph of it

Oh boy... a photograph? Of a subatomic particle?

we'd have to make a photograph of it using a sufficiently high frequency of light, otherwise we'd get a severe interference pattern.

I don't even...

thus knocking the original particle out of its path

This is the only part that made any sense.

If you're detecting a particle, you have to use another particle to do it, 'cause otherwise... how would you? So it's like finding out information about a car by blindly throwing other cars at it and measuring the collision: you're gonna affect the thing you're measuring by the act of measuring it.

Re:What I never understood about the uncertainty p (1)

StripedCow (776465) | more than 3 years ago | (#36337964)

So it's like finding out information about a car by blindly throwing other cars at it and measuring the collision: you're gonna affect the thing you're measuring by the act of measuring it.

The point was that you could detect the position of the car by using much lighter objects (or objects with less energy), e.g. ping pong balls, and by deducing the position of the car from the interference pattern.

Re:What I never understood about the uncertainty p (1)

Scrameustache (459504) | more than 3 years ago | (#36338044)

The point was that you could detect the position of the car by using much lighter objects (or objects with less energy), e.g. ping pong balls

Ok, re-substitute "car" back to photon. Your ping-pong ball is a substitution for what, and how are you measuring that?

Re:What I never understood about the uncertainty p (1)

Anonymous Coward | more than 3 years ago | (#36338060)

And how would you do that when the particle you're trying to measure IS the size of the smallest thing you can reliably throw at your target?

When measuring the path of a photon, you only have other... photons to throw at it. though there's some that have the idea that they can reverse proton smash to get smaller resolutions, (ie, smash a particle that you reliably know how it should explode, and measure the interactions of those sub-atomic particles with the particle in question) it's a LONG stretch to get there currently.

the car analogy only works if you keep in mind that the car is the smallest unit of matter in that universe. there is no smaller bits that you can pick up and toss around. a better analogy would be pool balls (where they're all the same size/weight) and you can not put anything onto the table. you have to (without looking) determine the vector's momentum and direction with nothing but other pool balls. try getting someone to shoot a ball across the table, and determine it's direction AND momentum using nothing but another pool ball (while blindfolded)

Re:What I never understood about the uncertainty p (2)

z3alot (1999894) | more than 3 years ago | (#36338662)

I think I can safely say that nobody understands quantum mechanics.

Richard Feynman, in The Character of Physical Law (1965)

That said, I think I can attempt to clarify some of your misunderstandings from my own understanding. In fact someone set me straight if I have any issues of my own :)

The entire notion of a point particle is essentially a classical approximation (as far as geometry goes). In fact, all the spatial information that can be known (ie not completely transparent to the rest of the universe) about a particle is completely contained in its wave function, complex valued defined at every point in space. But the wave function in time must satisfy the Schrödinger equation [wikipedia.org] , and it has been shown by people smarter than I that wave function solutions *must* satisfy the uncertainty principle.

Its easiest to consider what this means in one dimension. Solutions of the Schrödinger Equation are linear combinations of sinusoidal functions of all wavelengths and velocities (with the solution for a particular particle determined like with any differential equation by the spatial and temporal boundary conditions). This is immediately consistent with the wave description of light and matter, as a sinusoidal function has a definite velocity but its position is not defined at all (it looks like a wave :P). So how then can we get a localized particle, like those we apparently observe enough to create an entire classical theory around? Well it turns out that taking linear combinations of waves of differing velocities causes local areas of destructive and constructive interference, and one can mathematically construct what's known as a wave packet [wikipedia.org] . Btw, the time evolution of the wave packet in the picture on wikipedia is incorrect for solutions to the Schrödinger Equation: particle wave packets necessarily disperse over time depending on the represented wave velocities (don't quote me on that). This means the range of represented wave velocities actually has physical significance. Anyway there's a limit to how localized a wave packet can get, called a Gaussian wave packet. To achieve this limit, one has to sum over essentially every possible wave velocity.

So solutions of the Schrödinger Equation can be something with no localization at all and a perfectly well defined velocity, like a sinusoidal function, or with a very acute (but not perfect) localization achieved by an almost infinite range of velocities of component waves. In fact there is a very simple inequality [wikipedia.org] expressing the relationship between the smallness of the localization to the range of velocities (momenta, actually)...

So all that's not that bad. The real strangeness of QM comes with what observation does to the wave function of a particle. Somehow, the act of observation (something I am not knowledgeable enough to define, but examples of which are hitting it with a photon or having it excite the screen in the double slit experiment, or even covering up a slit thus knowing it must go through the other) "collapses" the wave function of a particle back into its most localized form. The probability distribution of the center of the new localized form is given by the product of the wave function with its complex conjugate just before the observation.

The interference pattern corresponds to the probability distribution of particles when they reach the screen behind the double slit. If I fired only one particle through the double slit, it would cause a single photon (probably) to be emitted from the screen, with its location determined by the probability distribution. We can see an interference pattern because we are firing a beam of particles, not just one at a time. The kicker from the experiment is if we observe the interference pattern (say by collecting billions of data points from electrons fired one at a time), information about which slit each electron went through (in particular) is transparent to the rest of the universe; it cant be determined from say where that electron struck the screen. This seems to be where your misunderstanding lies, at least in part. If we had observed which slit the electron went though (by covering up a slit, or by shining light on it and doing what scientists do), then the interference pattern would have disappeared.

Re:What I never understood about the uncertainty p (1)

z3alot (1999894) | more than 3 years ago | (#36338700)

tl;dr

A: why is this so non-intuitive???

B: Quantum mechanics! *winks knowingly*

A: ohhhhh

Re:What I never understood about the uncertainty p (1)

John Hasler (414242) | more than 3 years ago | (#36338956)

We can see an interference pattern because we are firing a beam of particles, not just one at a time.

But the "beam" can be so weak that there is never more than one particle in transit at a time.

Re:What I never understood about the uncertainty p (1)

z3alot (1999894) | more than 3 years ago | (#36339470)

Sorry I should have been clear. All I meant was that we wouldnt "see" the interference pattern with just a single electron since it just excites a single atom (talking about wavefunction collapse when it hits the screen). But thats exactly right the electron still interferes with itself and the probability distribution of where we see it is the same as the interference pattern.

Re:What I never understood about the uncertainty p (1)

colinrichardday (768814) | more than 3 years ago | (#36339504)

The wavelength of the light is a lower bound on the error of detecting the particle's position. Higher frequencies of light correspond to longer wavelengths, which yield higher lower bounds of error.

My bad (1)

colinrichardday (768814) | more than 3 years ago | (#36339550)

Higher frequencies of light correspond to shorter wavelengths. The only way to lower the fuzziness is to use higher frequencies, which has a greater effect on changing the momentum.

FULL TEXT (1)

Anonymous Coward | more than 3 years ago | (#36337866)

https://rapidshare.com/#!download|639tl|2460541193|Science-2011-Kocsis-1170-3.pdf|662|R~0

Experiment is about "Measurement" not UP (4, Interesting)

bkpark (1253468) | more than 3 years ago | (#36337880)

Averaging over many measurements won't allow you to "defeat" uncertainty principle, as uncertainty principle tells you the width of the distribution (of measurements). If you wanted to get a precise measurement of the center of that distribution, yes, you can take many averages and reduce the error on that (see error of the mean [wikipedia.org] ), but the width of the distribution (given by uncertainty principle), remains unchanged.

Reading the paper abstract:

A consequence of the quantum mechanical uncertainty principle is that one may not discuss the path or “trajectory” that a quantum particle takes, because any measurement of position irrevocably disturbs the momentum, and vice versa. Using weak measurements, however, it is possible to operationally define a set of trajectories for an ensemble of quantum particles. We sent single photons emitted by a quantum dot through a double-slit interferometer and reconstructed these trajectories by performing a weak measurement of the photon momentum, postselected according to the result of a strong measurement of photon position in a series of planes. The results provide an observationally grounded description of the propagation of subensembles of quantum particles in a two-slit interferometer.

It looks like the goal of experiment is to nail down (or get further in nailing down) what constitutes "measurement [wikipedia.org] ". But I'm still trying to figure out how this experiment is different from the standard QND [wikipedia.org] (which doesn't claim not to collapse the wavefunction as all measurements ought to).

There was already an experiment that could do it! (1)

Anonymous Coward | more than 3 years ago | (#36338634)

I already read about an experiment, where they managed to find the slit a photon went through, without doing a measurement on the photon itself, preserving its wave nature.

It was really ingenious (sorry, can't remember 100% of it):
They entangled the input photon with another one, which went on a parallel course outside the experiment.
Now the two slits had one of the two polarizations. So if the photon went through one slit, it got its polarization. And so did its entangled partner.
Then the photon ended up in the sensor. Which showed a nice wave distribution when the experiment was repeated often enough. Obviously, since no measurement happened.

BUT: Now that the photon had given us the result, its partner could *still* be measured! And thatâ(TM)s what they did.
This allowed them, to find out which slit the first photon went through, by only measuring it after it was too late to get all particle-ey. ;)

To me this is sheer genius. So simple. And so elegant.

P.S.: Measurement is already defined in the most exact way possible. It is the transfer of a structural property (=information) from one particle to another one. Which, if it doesn't happen via entanglement, has to happen trough the exchange of force particles. (Unless I'm misinformed, and entanglement also uses some force.)
I specifically said "one". Referring to one quantum.
There's nothing to nail down there. It's already fused to the ground. ;) And unless we have to throw away the standard model (which can, of course, never be guaranteed, since it's only a theory), thereâ(TM)s no way around it.

...and they concluded? (1)

Anonymous Coward | more than 3 years ago | (#36337894)

What?

Re:...and they concluded? (1)

Anonymous Coward | more than 3 years ago | (#36338092)

This was on Ars yesterday or so, http://arstechnica.com/science/news/2011/06/an-experiment-that-just-keeps-on-giving.ars

It's important to realize that these are not the trajectories of individual photons—instead they are more like probability clouds that tell you where photons are most likely to be found. And what do you know? Half the photons appear to have gone through one slit and half go through the other.

But, in fact, this is a lie. That photon still has to have gone through both slits. It is important to realize that a measurement has to produce a result. It is always going to find that the photon is somewhere, and that tells us very little about where it came from or where it is going to.

Re:...and they concluded? (2)

Ant P. (974313) | more than 3 years ago | (#36339304)

That they can reliably measure cats.

Re:...and they concluded? (1)

numbski (515011) | more than 3 years ago | (#36340288)

No. They have finally made the Heisenberg Compensator so they can subsequently uncouple it and free Moriarty from the holodeck. Duh.

Be vewy vewy quiet... (2)

excelsior_gr (969383) | more than 3 years ago | (#36337922)

...I'm hunting wavicles! Wehehehehehe!

What about the Heisenberg Compensator? (1)

Anonymous Coward | more than 3 years ago | (#36337998)

Easier if you ask me...

A Quick Comment (1)

Anonymous Coward | more than 3 years ago | (#36338186)

The goal of measurement is to find both the position and momentum of a photon so that they can plot a trajectory in order to predict the future speeds and positions. The uncertainty principle precludes exact measurement of both, but in this experiment they utilize a 'weak' type of measurement and by repeating the experiment they get averages of trajectories. This does not violate the uncertainty principle but does start to give average trajectories in contrasts to single dimensional data (ie position or momentum). It's not as sexy as saying that the uncertainty principle has been circumvented, but weak measurement can be used to give these averages which is a step up from the current techniques for some applications.

Here, robust statistics might be bad (1)

G3ckoG33k (647276) | more than 3 years ago | (#36338188)

Here, robust statistics might be bad. Normally, I would say, robust statistics is superior to the crap called "parametric statistics, based on the junk "arithmetic mean", etc.

Yet, I would guess that the few outliers of interest here would have been missed by the Buick version of statistics - the median. Hence, the AMC Pacer would win hands downs as it would steer away for any folly in its way.

Researchers see results & wave over collegues. (1)

leftie (667677) | more than 3 years ago | (#36338236)

...to see the results themselves, and blow the whole experiment.

How it would sound on Star Trek... (2)

MobileTatsu-NJG (946591) | more than 3 years ago | (#36339458)

By averaging over a great many photons passing through the apparatus, and only measuring the light patterns on a camera, the team was able to infer what paths the photons had taken. While they were able to easily observe the interference pattern indicative of the wave nature of light, they were able also to see from which slits the photons had come, a sure sign of their particle nature."

Just like over-inflating a balloon...

So What? (2)

Katchu (1036242) | more than 3 years ago | (#36339822)

So What? We are the average of the various quantum states of our constituent particles, at least in THIS universe.

I tat I taw an intafewance pattan (0)

Anonymous Coward | more than 3 years ago | (#36340006)

I'm conducting a government-funded zero-slit experiment.
Still waiting for results.
Will continue the experiment until government (taxpayer) money runs out.

Averages kill the theory? (0)

Anonymous Coward | more than 3 years ago | (#36341746)

What do you want? The result a theory that is correct on average at best?
As Richard Feynman pointed out in 1979.

It may require some time to follow these lectures, but they are awesomely interesting:

http://vega.org.uk/video/subseries/8

Lucky bastards (0)

Anonymous Coward | more than 3 years ago | (#36341860)

have conducted a two-slit experiment

/me laughs at the surrender monkeys. (0)

Anonymous Coward | more than 3 years ago | (#36341950)

So from reading this, I've gathered that Canada has bridged the basic gap of understanding fundamental physics, which up to this point were merely hypothetical theories based on quantum axioms and differential speculations, pertaining primarily to a photons path based on a set origin and designated destination - thus allowing them to fold time and space using a particles ascertained trajectory. Finally the copious amounts of spice (Frank H reference) they've been hording will come in handy!

(all in good fun, great thread)

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